Fluvial radiocarbon and its temporal variability during contrasting hydrological conditions

To study the significance of land use and catchment hydrology for the age of exported carbon, we measured the radiocarbon abundances in two mountainous streams in Germany. The Hassel catchment was characterized by wider-stretched riparian areas and by a significant contribution of arable land and pasture to land use compared to the Rappbode catchment. Although soil cultivation is considered to mobilize old carbon from deep soil layers, in six out of eight situations dissolved organic carbon (DOC) was younger in Hassel stream compared to Rappbode. In contrast, we estimated annual exports of radiocarbon on the basis of yield weighed, bulked biweekly samples and found that older DOC was delivered from Hassel compared to Rappbode catchment (352 years conventional radiocarbon age as well as carbon near to modern origin, respectively). This result characterized primarily the fluxes during wet conditions, which dominated the annual exports. In a winter base flow situation, we found that old (~2550 years) snow-bound organic carbon deriving from atmospheric deposition contributed 29–43 % to stream exports. Overall, there was significant intra-annual variability of stream 14C-DOC. Within the catchments, the standard deviations of ∆14C values (47–63 ‰) were comparable to the difference of annual exports between the contrasting catchments (46 ‰). Infrequent storm flow events should be included not only in export budgets but also in studies of carbon sources.